EP4264227A1

EP4264227A1 - Method for calibrating a particle sensor, particle sensor and device having a particle sensor

Info

Publication number: EP4264227A1
Application number: EP21839895.6A
Authority: EP
Inventors: Michael Foertsch; Stefan HENGESBACH; Jens GRIMMEL; Helge HATTERMANN
Original assignee: QAnt GmbH
Current assignee: QAnt GmbH
Priority date: 2020-12-18
Filing date: 2021-12-15
Publication date: 2023-10-25
Also published as: WO2022129158A1; KR20230119708A; US20230324276A1; JP2024503990A; DE102020216258A1; CN116601480A

Abstract

The invention relates to a method for calibrating a particle sensor (2), comprising: - focusing a light beam, in particular a laser beam (8), onto a calibration plane (KE) to generate a calibration intensity distribution (10), in particular a calibration focus, in the calibration plane (KE), wherein a calibration disc (1), on which contrast regions (2) for modulating the intensity (I) of the light beam, in particular of the laser beam (8), are formed, is provided in the calibration plane (KE); - moving the calibration disc (1) and/or the calibration focus (10) in the calibration plane (KE); - detecting at least one intensity signal (IT; IR) of the light beam, in particular of the laser beam (8), after it has passed through the calibration plane (KE); and - calibrating the particle sensor (2) by evaluating the at least one intensity signal (IT; IR). The invention also relates to a particle sensor (2) for carrying out the method and to a device having at least one such particle sensor (2).

Description

Method for calibrating a particle sensor, particle sensor and device with a particle sensor

The invention relates to a method for calibrating a particle sensor, a particle sensor and a device with at least one such particle sensor.

Particle sensors are used to characterize particles, for example to determine a particle position, a particle speed, a particle acceleration, a particle trajectory and/or a particle size. The characterization of particles is of great relevance for many sectors, such as the chemical, pharmaceutical or semiconductor industries.

Particle sensors based on optical measuring methods, in particular with laser radiation, are widely used in monitoring air and liquid purity. However, the accuracy of such particle sensors is impaired as soon as contamination accumulates on surfaces in the optical path of the laser beam or the parameters of the light beam or the laser parameters change. Thereby time-consuming recalibration of the system becomes necessary, which cannot always be carried out in the field.

US Pat. No. 3,885,415 describes a calibration device for the automatic calibration of an analysis system for measuring particle velocities. The calibration device comprises a disk which has contrasting areas on its surface for modulation of the light intensity. The contrast areas are moved in order to simulate the movement of particles that are introduced into a measuring cell during the measuring operation of the analysis system. The pane or the contrast areas can be introduced into an observation beam path of the analysis system for the calibration of the analysis system. The disk defines a calibration plane located near a surface where an image of the particles is formed.

object of the invention

The invention is based on the object of providing a method, a particle sensor and a device with a particle sensor which enable the particle sensor to be calibrated with high accuracy.

subject of the invention

This object is achieved by a method for calibrating a particle sensor, comprising: aligning, in particular focusing, a light beam, in particular a laser beam, on a calibration plane to generate a calibration intensity distribution, in particular a calibration focus, in the calibration plane, with a calibration disk being arranged in the calibration plane , on which contrast areas for modulating the intensity of the light beam, in particular the laser beam, are formed, moving the calibration disc and/or the calibration intensity distribution in the calibration plane, detecting at least one intensity signal of the light beam, in particular the laser beam, after passing through the calibration plane, and calibrating the Particle sensor by evaluating the at least one intensity signal, typically taking into account (known) Properties of the contrast areas.

In the method according to the invention, contrast areas on a calibration disk are used for calibrating the particle sensor, which have known properties, e.g. a known size or a known diameter. The contrast areas serve to simulate particles or these form calibration particles which are detected by the particle sensor during calibration operation and which differ from the surroundings of the contrast areas on the calibration disk by at least one (optical) property. The surroundings of the contrast areas can be formed, for example, by the substrate of the calibration disc. The contrast areas can, for example, at least partially scatter, absorb, diffract or reflect the light or laser beam. The area surrounding the contrast areas, for example in the form of the substrate, differs as much as possible from the optical properties of the contrast areas and can, for example, (almost completely) transmit the light or laser beam. A strong difference between the contrasting areas and the surroundings also arises, for example, when the contrasting areas reflect the light or laser beam, while the surroundings of the contrasting areas absorb the light or laser beam.

For the calibration, use is made of the fact that the contrast areas have known properties, e.g. a known size/area or geometry and known optical properties with regard to absorption, reflectivity, scattering behavior, etc. The movement of the calibration disk or the calibration intensity distribution in the calibration plane is also specified during the calibration and is therefore known. A calibration particle with known properties can therefore be moved along a known trajectory by moving the calibration disk or the calibration intensity distribution. The particle sensor can therefore be calibrated, e.g. with regard to the measured variables particle size, particle material, particle trajectory, particle speed, particle acceleration, etc.

For the purposes of this application, a calibration of the particle sensor also includes a check of an already existing calibration of the particle sensor Roger that. In the event that the check reveals that the existing calibration is faulty, which can be due, for example, to contamination of surfaces in the beam path of the light or laser beam or to a change in the parameters of the light or laser beam (see above ), the particle sensor can be recalibrated.

The particle sensor characterizes the calibration particle in the same way as a particle that passes through a measurement volume of the particle sensor, i.e. the calibration disk simulates a particle flying through the particle sensor. In the event that the light or laser beam strikes a calibration particle or a contrast area during the movement of the calibration disc or the calibration intensity distribution, this typically causes a weakening of the detected intensity signal. The attenuation of the intensity signal is typically greater, the larger the area of the calibration particle. In particular, the attenuation of the intensity signal can be proportional to the area of the calibration particle.

The light or laser beam is aligned to the calibration level. The light beam or laser beam that generates the calibration intensity distribution in the calibration plane can be a collimated light beam or laser beam, for example. In the event that particles with a comparatively small particle diameter are to be characterized, it is advantageous to focus the light or laser beam in the calibration plane. In this case, the calibration intensity distribution in the calibration plane forms a calibration focus.

The calibration method described above can be used with a large number of particle sensors that function on the basis of optical radiation, in particular laser radiation. For example, the calibration method can be used in a sensor arrangement or in a particle sensor, as is described in DE 10 2019 209 213.6, which is incorporated by reference in its entirety into the content of this application. In the particle sensor described there, a mode conversion device is used to generate a field distribution of the laser beam, which has a different combination of a local intensity and a field distribution at each position has local polarization direction of the laser beam. An analyzer optic is used to determine polarization-dependent intensity signals of the field distribution of the laser beam that has passed through a measurement area. The particles are characterized using the polarization-dependent intensity signals, using the fact that the field distribution or the optical modes of the field distribution have a clear correlation between the local intensity distribution and the local polarization direction. It goes without saying, however, that other types of particle sensors, which are based on optical radiation, for example laser radiation, or the attenuation of optical radiation, for example laser radiation, when passing through a measurement volume, can also be calibrated using the calibration method described here be able.

In one variant, the calibration disk is arranged in a calibration plane within a measurement volume through which the particles pass when the particle sensor is measuring. In this variant, the calibration disc is inserted from the outside into the measurement volume through which the particles flow during measurement. In this case, the calibration takes place outside of the measuring operation in order to calibrate the particle sensor before commissioning or during maintenance. This variant requires that the measuring volume is accessible from the outside in order to position the calibration disc, which is not always possible.

In an alternative variant, the calibration intensity distribution, in particular a calibration focus, in the calibration plane and a measurement intensity distribution, in particular a measurement focus, are imaged on one another in a measurement plane. The measurement plane is formed in a measurement volume through which the particles pass during measurement operation of the particle sensor. In this case, the calibration disc is not introduced into the measurement volume, but is arranged in the beam path of the light or laser beam at a distance from the measurement volume. The light or laser beam can be focused in the measurement plane so that the measurement intensity distribution forms a measurement focus, but this is not absolutely necessary. The calibration intensity distribution or the calibration focus and the measurement intensity distribution or the measurement focus can be imaged on one another with the aid of a beam shaping device in the form of imaging optics. By imaging optics, an intensity distribution or a Double focus generated with comparable beam parameters (beam radius, etc.). For example, the calibration disk can be placed in one of the two foci (calibration focus), the second focus (measurement focus) is located in the measurement volume through which the particles to be characterized flow. By knowing the beam radii of the calibration intensity distribution or the calibration focus and the measurement intensity distribution or the measurement focus, the size of the particles in the measurement volume can be calculated after the calibration using the calibration particles or the contrast areas of the calibration disk. The particle size of a particle is understood to mean the cross-sectional area of a particle in the measurement plane.

In this variant, the calibration disc can be integrated into the particle sensor so that the calibration can be checked and repeated when the particle sensor is used in the field. When the particle sensor is measuring, the calibration disc is not moved and is either removed from the beam path of the light or laser beam or positioned in such a way that the light or laser beam is not weakened too much. For example, the calibration disc can be positioned in such a way that the light or laser beam passes through the calibration disc on a transparent area of the substrate on which no contrast area is formed.

In this variant, not only does the measurement intensity distribution or the measurement focus in the measurement plane form a kind of "virtual sensor surface" that enables remote characterization of the particles without having to position the sensor at the particle location, but also the calibration intensity distribution or the calibration focus in the calibration level enables a calibration without bringing the calibration particles into the measurement level or into the measurement volume. In this variant, a remote diagnosis or a remote calibration of the particle sensor is thus made possible.

In a further development of this variant, the calibration disc is arranged in a housing that is separate from the measuring volume. It is favorable to place the calibration disk in a volume that is hermetically separated from the measurement volume to be arranged in order to protect the calibration disc from contamination by the particles that flow through the measuring volume.

In a further variant, the contrast areas on the calibration disk have different surface areas. The different surface areas or sizes of the contrast areas simulate calibration particles with different, previously known particle sizes or particle diameters. The size of the contrast areas is adapted to the type of particle to be characterized or to the type of particle sensor. The calibration with contrast areas or calibration particles of different sizes on the calibration disc enables a high level of accuracy in the subsequent particle measurement over a wide size range.

In a further variant, the contrast areas are formed by microstructures on the surface of a substrate, in particular a wafer. In this case, the calibration disc can have a substrate, e.g. made of glass or a crystal, on which contrast areas are formed in the form of microstructures with known structure sizes, which act in the manner of calibration particles. The microstructures can be applied, for example by microlithography, to the surface of a transparent substrate, for example in the form of a glass wafer. The contrast areas can form at least partially absorbing or reflecting or scattering structures for the light or laser beam. Due to the microstructuring, both the shape and the size or the surface area of the contrast areas are known with high precision, so that they can be used as a calibration standard. The microstructuring can take place on a layer, e.g. on a metal layer, which is applied to the substrate, but it is also possible for the substrate itself to be microstructured. For the comparison of the sizes of the contrast areas produced by the microstructuring, the calibration, which is carried out with the aid of the calibration disk, can be verified, if necessary, with the aid of standard particles of a defined size, for example with the aid of polystyrene balls of a known size.

In an alternative variant, the contrast areas are formed by calibration particles, in particular with a predetermined particle size Calibration particles preferably have at least two groups of calibration particles with different optical properties, the calibration particles of the at least two groups being formed in particular from different materials. The different optical properties can be, for example, a different (complex) refractive index, which means that the calibration particles absorb, reflect, scatter and/or diffract the light or the laser radiation differently. The use of calibration particles made of different materials makes it possible to carry out the calibration of the particle sensor for the detection of particles made of different materials or material classes without different calibration discs being required for this purpose.

The materials from which the calibration particles of a respective group are formed can be, for example, metals (eg steel), sand (eg SiO ₂ ), plastic (eg polystyrene or latex), ceramics, etc., since these Materials typically differ significantly in their optical properties. The three material classes of metals, sand and plastics are typical components of dust.

In this case, the calibration particles can, for example, be applied to the calibration disc with a specified distribution or with a specified pattern (or in specified surface areas or fields), but the calibration particles of the different groups can also be applied statistically distributed (see below) if necessary. . It is of course also possible to apply only one type of calibration particle, possibly with different predetermined particle sizes, to the calibration disc.

In a development of this variant, the calibration particles are applied to the surface of a substrate in a statistically distributed manner and/or the calibration particles are formed by self-structuring on the surface of the substrate.

In the first case, the size of the calibration particles is known, but the exact distribution on the surface is not known in advance; instead, the calibration particles are statistically distributed when they are applied to the surface. the In this case, calibration particles, ie particles with a known size, can be suspended in a low concentration in liquid, transparent adhesive, for example, and applied as a thin layer to the calibration disk or to the substrate. After the adhesive has dried, the calibration particles are randomly distributed over the calibration disc. In this variant, in particular, spherical, monodisperse particles of known size are suitable as calibration particles, eg polystyrene-latex beads, which are established as the standard for particle size measurement and are available with a narrow size distribution. Calibration particles of this type are used, for example, to calibrate particle counters in accordance with the ISO 21501 standard, see the link "de.wikipedia.org/wiki/ISO_21501".

In the second case, the adhesive or a photoresist is applied to the disc-shaped substrate by means of a dispenser. Before curing (especially with UV light or in the oven), this photoresist or adhesive forms spherical segments up to hemispheres depending on its viscosity due to the surface tension. The three-dimensional structure of the calibration particles generated in this way, i.e. by self-structuring, enables the scattering behavior of particles with a spherical surface to be approximated and is therefore particularly suitable for particle sensors that are based on the principle of scattered light measurement. The structural size of the calibration particles generated in this way depends on the process parameters, e.g. during the curing of the photoresist or adhesive, and can therefore be assumed to be known given the process parameters. The disk-shaped substrate can also be cleaned, coated or laser-structured before the adhesive is applied, resulting in a hydrophilic or hydrophobic surface for the customized shaping of the droplets.

In a further variant, the calibration disc is automatically rotated and/or automatically displaced when moving in the calibration plane.

The calibration disk can be turned or rotated around a specified axis of rotation in order to move it in the calibration plane and to simulate the movement of a particle. The automated rotation of the calibration disc can be done with the help of an electric motor, for example. With a calibration disk of suitable diameter, the Calibration particles in this case almost linearly through the light or laser beam of the particle sensor and cause a weakening of the intensity of the light or laser beam, which is usually proportional to the area of the contrast area or the calibration particle. In this case, the calibration disc is typically circular and can have a diameter of between 10 mm and 100 mm, for example. For example, the calibration disc can be rotated at a frequency between 0.1 Hz and 200 Hz. For example, between 1 and 400 calibration particles per revolution can be attached to the calibration disc. The calibration particles can be round, for example, and have been applied by a lithographic process. The light or laser beam can have a diameter of between 20 μm and 200 μm at the calibration intensity distribution or at the calibration focus, for example. The diameter of the calibration particles is typically between 0.05% and 100% of the focus or beam diameter.

The calibration disc can also be moved linearly in the calibration plane. In this case, for example, a piezo drive or a piezo crystal or a linear drive can be used for automated displacement. The use of a piezo drive to move the calibration disc has proven to be advantageous, since it works wear-free and therefore does not produce measurement errors due to abrasion that could otherwise be deposited on the calibration disc. The frequency of the displacement of the calibration disc can be of the order of between 1 Hz and 100 Hz, for example. In this case, the calibration disc can have a rectangular geometry, for example.

Alternatively or additionally, the calibration intensity distribution or the calibration focus of the light or laser beam can also be moved in the calibration plane. To move the calibration intensity distribution, the light or laser beam can carry out a scanning movement, for example by being deflected using mirror scanners or scanner mirrors, using an acousto-optical modulator, etc., in order to vary the position of the calibration intensity distribution in the focal plane . In this case, the calibration disk can be stationary, but this is not necessarily the case, ie it can also be a superimposed movement of the calibration disk and the Calibration intensity distribution done.

Another aspect of the invention relates to a particle sensor, comprising: a transmitter, which has a light source for generating a light beam, in particular a laser source for generating a laser beam, optics for aligning, in particular for focusing, the light beam, in particular the laser beam, on a Calibration plane for generating a calibration intensity distribution, in particular a calibration focus, in the calibration plane, with a calibration disc being arranged in the calibration plane, a movement device for moving the calibration disc and/or the calibration intensity distribution in the calibration plane, a receiver having a detector for detecting at least one intensity signal of the Light beam, in particular the laser beam, after passing through the calibration level, and an evaluation device for evaluating the at least one intensity signal for calibrating the particle sensor in a Ka libration operation of the particle sensor.

As described above in connection with the method, the movement of particles is simulated in the calibration mode of the particle sensor with the aid of the contrast areas. If the size of the contrast areas or the calibration particles is known, the particle sensor can be calibrated on the basis of the detected intensity signal(s). During the calibration, the intensity signals recorded in each case can be stored as characteristic for a specific particle size or for specific particle speeds and/or particle trajectories and can be used as a calibration standard. In the measurement mode of the particle sensor, this calibration standard can be used to characterize particles that pass through a measurement volume.

With the particle sensor described here, the particles can be characterized in transmission, ie the light or laser beam is emitted by the transmitter and passes through the measurement volume in transmission. However, it is also possible for the particle sensor to characterize the particles by scattering and/or absorbing the light or laser beam on the particles. In the last two cases, the reflected or scattered light or laser beam is on detected by the detector of the receiver.

In one embodiment, the calibration disk is arranged in a calibration plane that is located in a measurement volume through which the particles pass when the particle sensor is in measurement mode. As was described above, the calibration disk is typically only arranged in the measurement volume in the calibration mode in this case. In the measurement mode of the particle sensor, the calibration disc is either automatically or manually removed from the measurement volume.

In an alternative embodiment, the optics of the particle sensor comprises imaging optics for imaging the calibration intensity distribution in the calibration plane and a measurement intensity distribution, in particular a measurement focus, in a measurement plane on one another, the measurement plane being formed in a measurement volume that is formed by the particles in a measurement operation of the particle sensor is passed through. In this embodiment, the calibration disk is usually permanently integrated into the particle sensor, typically into the transmitter of the particle sensor. In this case, the calibration disc can be automatically introduced into the beam path of the light or laser beam, if necessary before the start of the calibration operation, but it is also possible for the calibration disc to be permanently arranged in the beam path of the laser beam. In this case, the calibration disk is positioned in such a way that it interferes with the measurement process as little as possible. In this case, for example, the position at which the light or laser beam strikes the calibration disc can be at a point where no contrast area is arranged.

In a further embodiment, the transmitter has a housing with an exit window and the receiver has a housing with an entrance window, between which the measurement volume is formed. In this case, the calibration disc is typically located in the housing of the transmitter or receiver and hermetically separated from the measurement volume. This is beneficial in order to prevent contamination of the calibration disk by the particles that pass through the measurement volume. The arrangement of the calibration disk in the receiver is preferred because of soiling of the entry or exit window a power reduction of the light or laser beam can occur. In this case, the calibration can be carried out with the already reduced power and with the possible optical disturbances of the light or laser beam due to the dirt on the entry or exit window.

The calibration disc can be designed in the manner developed above in connection with the method. For example, the contrasting areas can have different surface areas. The contrast areas can also be designed to at least partially scatter, absorb or reflect the light or laser beam. The contrast areas on the calibration disc can be formed by microstructuring. It is also possible for the contrast areas to be formed by calibration particles with a predetermined particle size, which may be applied statistically distributed to the surface of a substrate of the calibration disk, or for groups of calibration particles with different properties to be provided on the calibration disk.

In a further embodiment, the movement device is designed to rotate the calibration disk in the calibration plane. In this case, the movement device can be designed, for example, as an electric motor, which acts on a rotary axis on which the calibration disk is fastened.

In a further embodiment, the movement device is designed to move the calibration disk in the calibration plane. In this case, the movement device can be designed, for example, as a piezo actuator, which acts, for example, on a lateral edge of the calibration disc in order to move it in the calibration plane. The piezo actuator enables the calibration disk to move without abrasion in the calibration plane.

Another aspect of the invention relates to a device, in particular an EUV radiation generating device, comprising: a measuring chamber to which particles, for example in a target area, can be fed, and a particle sensor, which is designed as described above, for characterizing the particles in the measuring chamber. As described above, the particle sensor can be used in a large number of fields of application in order to characterize solid, liquid or gaseous particles or particle streams. The measuring chamber can be designed, for example, to receive or flow through a gas or a liquid. In the first case, the device can be used, for example, to measure air purity, for example to characterize soot particles or to measure fine dust. It is also possible to characterize powders, for example with regard to their powder or grain size distribution, for example in the case of powders that are used for additive manufacturing (3D printing) or in the case of building materials, for example cement. The characterization of particles in liquids is also possible, eg the characterization of particles in the form of bacteria in milk. In all of these applications or devices, machine learning can be used to characterize the particles or to carry out the calibration. In the case of machine learning, test data is generated and a suitable AI, for example in the form of a neural network or the like, is trained.

The particle sensor can also be used in the semiconductor industry. For example, the particle sensor can be used to characterize particles or particle flows in a measuring chamber in the form of a vacuum chamber of an EUV radiation generating device. Such an EUV radiation generating device generally has a driver laser arrangement for generating a driver laser beam and a beam feed device for feeding the driver laser beam to the vacuum chamber described above. The driver laser beam is focused in a target area of the vacuum chamber, in which a target material in the form of tin particles or tin droplets is introduced or supplied. When irradiated with the driver laser beam, a respective particle changes to a plasma state and emits EUV radiation in the process. The particles of the target material that are guided to the target area, as well as the particles that are generated when the laser beam hits the target material (when the target material is atomized), can be characterized using the particle sensor described above. Further advantages of the invention result from the description and the drawing. Likewise, the features mentioned above and those listed below can each be used individually or together in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather have an exemplary character for the description of the invention.

Show it:

1a,b schematic representations of a calibration disc, which is used to calibrate a particle sensor and is designed for rotation or displacement in a calibration plane,

Fig. 2a, b schematic representations of the calibration disk from Fig. 1 a,b, which is arranged in a calibration plane in a measuring volume of the particle sensor,

3 shows a schematic representation of a particle sensor which has imaging optics for imaging a calibration focus in a calibration plane and a measurement focus in a measurement plane on one another, the measurement plane being formed in a measurement volume of the particle sensor, and

4 shows a schematic representation of an EUV radiation generating device which has a particle sensor for characterizing particles in a vacuum chamber.

In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

1a and FIG. 1b each show a calibration disc 1 for calibrating a particle sensor 2, which is shown in FIGS. 2a, b and in FIG. The calibration disc 1 has a transparent substrate 3 in the form of a crystal wafer, a glass wafer or a plastic disc. On the substrate 3, more precisely on a planar surface of the substrate 3, are in the example shown circular contrast areas 4 formed. The contrast areas 4 have a previously known size and geometry and are used as calibration particles for calibrating the particle sensor 2.

The particle sensor 2 shown in FIGS. 2a, b and in FIG. 3 has a transmitter 5 and a receiver 6 . The transmitter 5 includes a laser source 7 which is used to generate a laser beam 8 . Instead of a laser source 7, another type of light source that generates a light beam or optical radiation that is not laser radiation can optionally be used. The light source can be an LED or the like, for example.

In the example shown, the laser source 7 is a diode laser that has a laser wavelength that is selected from a wavelength range between approximately 250 nm and approximately 1600 nm, depending on the application. The laser beam 8 is focused with the aid of optics in the form of focusing optics 9 onto a calibration focus 10 in a calibration plane KE, in which the calibration disk 2 is arranged. The calibration focus 10 forms a calibration intensity distribution in the calibration plane KE. When passing through the calibration disc 1, more precisely the contrast areas 4, the laser beam 8 strikes the contrast areas 4 and its intensity I is modulated. The modulation of the intensity I can be realized, for example, by a reduced transparency of the contrast areas 4 compared to the substrate 3, by a modified refractive index of the contrast areas 4 compared to the substrate 3, . . .

In the example shown in FIG. 1a, the contrast areas 4 are formed from a material that absorbs the laser beam 8, while the substrate 3 of the calibration disc 1 is formed from a material that is transparent to the wavelength of the laser beam 8. If the laser beam 8 strikes a respective contrast area 4, the intensity I of the laser beam 8 decreases as it passes through the calibration disk 1.

The calibration disk 1 shown in FIG. 1a or the calibration plane KE is arranged in a measuring volume 11 of the particle sensor 2 shown in FIG. 2a. The receiver 6 of the particle sensor 2 of FIG. 2a is along the direction of propagation Z (Z direction of an XYZ coordinate system) along a line of sight to the laser source 7 or to the receiver 6 and has a detector 12 which serves to detect an intensity signal l _T of the laser beam 8 transmitted by the calibration disc 1 . An evaluation device 13 is used to evaluate the intensity signal l _T of the laser beam 8.

In contrast to the calibration disc 1 shown in FIG. 1a of FIG. 1b, the contrast areas 4 are formed from a material that reflects the wavelength of the laser beam 8. The calibration disk 1 shown in Fig. 1b is arranged in a calibration plane KE of the particle sensor 2 shown in Fig. 2b, which differs from the particle sensor 2 shown in Fig. 2a essentially in that the receiver s is at an angle to the propagation direction of The laser beam 8 generated by the laser source 7 is arranged, so that the detector 12 detects an intensity signal l _R of the laser beam 8 reflected at the contrast regions 4 . It is also possible that the detector 12 of the particle sensor 2 from FIG.

The focusing of the laser beam 8 in the calibration plane KE shown in FIGS. 1a, b serves to generate a small beam diameter of the laser beam 8 in the calibration plane KE. If necessary, focusing can be dispensed with if comparatively large particles are to be detected with the particle sensor 2 . In this case, the laser beam 8 can optionally be collimated or even radiated divergently onto the calibration plane KE. In this case, instead of the focusing optics 9, another type of optics, for example collimating optics, is used.

In order to form a calibration standard that is as accurate as possible for the calibration of the particle sensor 2, the contrast areas 4 are designed in the form of microstructures in the example shown in FIG. 1a. The microstructures are formed by the microlithographic structuring of a metal layer which was applied to the surface of the substrate 3 and which, with the exception of the circular contrast areas 4, was removed from the surface of the calibration disc 1 again during the microstructuring. The microstructuring allows the size of the contrast areas 4 and the calibration particles and specify their distances from each other precisely in order to increase the precision in the calibration of the particle sensor 2. In the example shown, the contrast areas 4 have surface areas of different sizes or diameters of different sizes in order to simulate particles of different sizes and in this way to enable the accuracy of the particle sensor 2 for different particle sizes, in particular over a wide size range. As an alternative to producing the contrast areas 4 by microstructuring, they can also be formed on the calibration disk 1 by ultra-short pulse processing, etc.

In the example shown in FIG. 1b, the contrast areas are formed by calibration particles 4, which are applied to the surface of the substrate 3 with a predetermined distribution (in a grid). The calibration particles 4 of adjacent columns of the grid-like arrangement differ from one another by their optical properties, in the example shown by their (complex) refractive index, ie they form different groups of calibration particles 4. In FIG. 1 b the groups of calibration particles 4 are made of different materials educated. 1b shows, by way of example, a first group 4a of calibration particles 4 made of a metallic material, more precisely steel, a second group 4b of calibration particles 4 made of sand (SiO ₂ ), and a third Group 4c of calibration particles 4, which is formed from a plastic (eg, polystyrene or latex). Due to the different materials of the calibration particles 4, the particle sensor 2 can be calibrated for the characterization of particles from different particle materials.

It is also possible for the contrast areas 4 on the calibration disc 1 to be formed by calibration particles which are applied to the surface of the substrate 3 in a statistically distributed manner. In this case, the contrast areas 4 are calibration particles, ie particles with a known size. To apply the calibration particles 4, they can be suspended, for example, in a low concentration in liquid, transparent adhesive and applied as a thin layer to the calibration disk 1 or to the substrate 3. After the adhesive has dried, the calibration particles 4 are random distributed over the calibration disc 1. In this case, in particular, spherical, monodisperse particles of known size, eg polystyrene-latex beads, are suitable as calibration particles 4 .

The adhesive or a photoresist can also be applied to the disc-shaped substrate 3 by means of a dispenser. Before curing (especially with UV light or in the oven), this photoresist or adhesive forms spherical segments up to hemispheres, which form the calibration particles 4, depending on its viscosity due to the surface tension. The three-dimensional structure of the calibration particles 4 produced in this way enables the scattering behavior of particles with a spherical surface to be approximated and is therefore particularly suitable for a particle sensor 2 which is based on the principle of scattered light measurement. The structural size of the calibration particles 4 produced in this way depends on the process parameters, e.g. during the curing of the photoresist or adhesive, and can therefore be assumed to be known given the process parameters. The disk-shaped substrate 3 can additionally be cleaned, coated or laser-structured before the adhesive is applied, so that a hydrophilic or hydrophobic surface results for the adapted shaping of the droplets.

The particle sensor 2 from FIGS. 2a, b and from FIG. 3 also has a movement device 14a, 14b in order to move the calibration disc 1 in the calibration plane KE during the calibration. In the case of the particle sensor 2 shown in FIG. 2a, the movement device 14a is an electric motor which has a motor hub 15 which serves as the axis of rotation for the calibration disk 1 of FIG. 1a. The circular calibration disk 1 shown in FIG. 1a is rotatably mounted and has a central bore in which the motor hub 15 of the electric motor 14a engages.

For the calibration of the particle sensor 2, the calibration disc 1 from FIG. The calibration disc 1 of FIG. 1a has a diameter that can be between about 10 mm and about 100 mm. With one revolution of the calibration disk 1 if necessary, only a single contrast area 4 can be run through. As a rule, however, a plurality of contrast areas 4 are distributed in the circumferential direction over the calibration disc 1; In particular, several hundred contrast areas 4 can be attached to the calibration disk 1 and pass through the calibration focus 10 during one rotation of the calibration disk 1 . The calibration disk 1 can be rotated, for example, at a frequency between 0.1 Hz and 200 Hz. A typical order of magnitude for the diameter of the laser beam 8 at the calibration focus 10 is between approximately 10 μm and approximately 5 mm, for example between approximately 40 μm and approximately 100 μm.

In the example shown in FIG. 2b, the movement device 14b is a piezo actuator which acts on a lateral edge of the calibration disk 1 from FIG. 1b in order to move it along a displacement direction Y in the calibration plane KE. The calibration disc 1 of FIG. 2b has a rectangular geometry to simplify displacement in the calibration plane KE. A particle trajectory of the contrast regions 4 or the calibration particles can also be simulated when the calibration disc 1 is displaced in the calibration plane KE. The piezo actuator 14a can, for example, move the calibration disk 1 at a (adjustable) frequency between 1 Hz and 100 Hz in the calibration plane KE and thereby deflect the calibration particles or the contrast areas 4 . The movement can in particular take place in a targeted manner within a specific group 4a-c of calibration particles 4 or along a specific column of the grid-shaped pattern on the calibration disk 1.

As an alternative or in addition to moving the calibration disc 1 in the calibration plane KE, the calibration focus 10 can also be moved in the calibration plane KE. For this purpose, the transmitter 5 can have a movement device, for example in the form of a scanner device, which includes one or more scanner mirrors, an acousto-optical modulator, a polygon mirror, etc. for moving the laser beam 8 and thus also the calibration focus 10 in the calibration plane KE. In the particle sensor 2 shown in FIGS. 2a, b, the calibration disc KE is arranged outside of the measuring operation in the measuring volume 11 through which particles pass during the measuring operation. For this purpose, the calibration disk KE can be introduced into the measuring volume 11 automatically or, if necessary, manually.

In the case of the particle sensor 2 shown in FIG. 3, the calibration can take place during ongoing operation (during measurement operation). In contrast to the particle sensor 2 shown in FIGS. 2a, b, the calibration plane KE in the particle sensor 2 shown in FIG Example, rather in the beam direction of the laser beam 8 at a distance from a measurement plane ME, which is formed in the measurement volume 11. The particle sensor 2, more precisely the receiver 6 of the particle sensor 2, has imaging optics 17 for imaging the calibration focus 10 in the calibration plane KE and a measurement intensity distribution in the form of a measurement focus 16 in the measurement plane ME. The imaging optics 17 serve to generate a double focus with comparable beam parameters (beam radius, etc.) in the calibration plane KE or in the measurement plane ME. For this purpose, the imaging optics 17 can comprise, for example, a collimation device and a focusing device, which can be embodied, for example, in the form of lenses or (curved) mirrors.

In the particle sensor 2 shown in FIG. 3, focusing optics 9 of the transmitter 5 serve to focus the laser beam 8 at the measuring focus 16 in the measuring plane ME. In principle, it is possible to swap the roles of the calibration plane KE and the measurement plane ME in the particle sensor 2 shown in FIG. In this case, the focusing optics 9 are used to focus the laser beam 8 onto a calibration focus 10 in the calibration plane KE. The calibration disc 1 is arranged in the transmitter 5 and not in the receiver 6, which has turned out to be rather unfavorable for the calibration of the particle sensor 2.

In principle, it is possible for the calibration disk 1 to remain permanently in the beam path of the laser beam 8 in order to carry out calibration on the fly to enable measurement. This makes sense if the measurement (eg for lumps in a fluid) should not be interrupted so that a 100% check can be carried out. In this case, it is necessary to distinguish the particles P, which pass through the measurement volume 11, from the calibration particles 4 during the ongoing measurement. This is possible, for example, if the calibration particles 4 differ from the particles P in the measurement volume 11 in at least one optical property. Such a distinction is also possible with the same optical properties, provided that the arrangement of the calibration particles 4 on the calibration disc 1 is known, provided that there are not too many coincidences of particles P in the measurement volume 11 and on the calibration disc 1. The pattern in which the calibration particles 4 are arranged on the calibration disc 1 can be specifically selected such that the time sequence of the intensity signals caused by the calibration particles 4 can be distinguished from the intensity signals of the particles P to be measured during the evaluation in the evaluation device 13 .

Alternatively, the calibration disc 1 can be automatically introduced into the beam path of the laser beam 8 in order to carry out the calibration and the calibration disc can be removed from the beam path of the laser beam again as soon as the calibration is complete. The movement device 14b can be used to move the calibration disk 1 in and out of the beam path, but it is also possible for a further movement device to be provided in the transmitter 5 for this purpose.

By knowing the radii of the laser beam 8, the calibration intensity distribution or the calibration focus 10 and the measurement intensity distribution or the measurement focus 16, the size of the particles P in the measurement volume 11 can be calculated after the calibration. It goes without saying that, in addition to the size of the particles P, other measured variables of the particles P can also be calibrated. These measured variables can be the particle position, the particle speed, etc., for example. The calibration of these measured variables is possible because the calibration disk 1 is moved in the calibration plane KE with the aid of the respective movement device 14a, 14b. The characterization of the particles P (and the calibration particles 4) takes place in the evaluation device 13 of the receiver 6 of the particle sensor 2. The detector 12 for detecting the intensity signals l _T , l _R of the laser beam 8 can be a spatially resolving detector, for example a CCD detector, but this is not mandatory. For example, the detector 12 can be designed for the non-spatially resolved detection of a plurality of polarization-dependent intensity signals, which enable the characterization of the particles P, as described in the patent application DE 10 2019 209 213.6 cited above. In this case, the detector 12 can have, for example, a plurality of photodiodes.

The particle sensor 2 described in connection with FIGS. 2a, b and FIG. 3 can be used to characterize particles P in a large number of different applications. Such an application is described in more detail below with reference to an EUV radiation generating device 30 by way of example with reference to FIG. 4 . The EUV radiation generating device 30 comprises a beam source 31, an amplifier arrangement 32 with three optical amplifiers or amplifier stages 33a-c, a beam guiding device 34, not shown in detail, and a focusing device 35. The focusing device 35 serves to generate a beam generated by the beam source 31 and from the Amplifier assembly 32 to focus amplified driver laser beam 31a at a target area 36 in a vacuum chamber 38 in which particles P are introduced. The particle P or an individual drop of tin serves as the target material and is irradiated by the driver laser beam 31a. In this case, the tin drop changes into a plasma state and emits EUV radiation, which is focused by means of a collector mirror 37 . In the example shown in FIG. 4, the collector mirror 37 has an opening for the laser beam 31a to pass through. In the example shown, the beam source 31 has two CO ₂ lasers in order to generate a pre-pulse and a main pulse, which are amplified together in the amplifier arrangement 32 and focused on the target area 36 . The beam source 31, together with the amplifier arrangement 32, forms a driver laser arrangement 39 of the EUV radiation generating device 30. As can also be seen in FIG. 4, the transmitter 5 and the receiver 6 of the particle sensor 2 are attached to the vacuum chamber 38 so that the vacuum chamber 38 forms a measuring chamber for the particle sensor 2 . The measurement volume 11, in which the measurement plane ME is formed, runs through the target area 36 with the particles P in the form of tin droplets. With the help of the particle sensor 2, the particles P or their movement to the target area 36 can be examined and their movement or trajectory can be determined. The size of the particles P or the size of smaller particles generated by the driver laser beam 31a when a respective tin droplet is atomized can also be determined with the aid of the particle sensor 2 . The trajectories or the speed of the particles P generated during atomization can also be detected using the particle sensor 2 .

As can also be seen in FIG. 4, the transmitter 5 is shielded from the environment by a housing 23 . Correspondingly, the receiver 6 is also shielded from the environment by a housing 24 . An exit window 21a is formed on the housing 23 of the transmitter 2 for the exit of the laser beam 8 . An entry window 21b for entry of the laser beam 8 into the receiver s after passing through the measurement volume 11 is correspondingly formed on the housing 24 of the receiver 6 . The windows 21a, b make it possible to shield the transmitter 5 and the receiver 6 from the environment, so that the particle sensor 2 can be used to detect different liquid, gaseous or solid media. In the case of the particle sensor 2 shown in FIG. 4 , the calibration disc 1 is therefore also shielded from the measuring volume 11 by the housing 23 of the transmitter 5 and is thus protected from the particles P which pass through the measuring volume 11 .

Claims

25

Claims Method for calibrating a particle sensor (2), comprising:

Aligning, in particular focusing, a light beam, in particular a laser beam (8), on a calibration plane (KE) to generate a calibration intensity distribution (10), in particular a calibration focus, in the calibration plane (KE), with a calibration disc ( 1) is arranged on the contrast areas (2) to

Modulation of the intensity (I) of the light beam, in particular of the laser beam (8), are formed,

Moving the calibration disk (1) and/or the calibration intensity distribution (10) in the calibration plane (KE), detecting at least one intensity signal (l _T ; IR) of the light beam, in particular the laser beam (8), after passing through the calibration plane (KE), such as

Calibrating the particle sensor (2) by evaluating the at least one intensity signal (l _T ; l _R ). Method according to claim 1, in which the calibration disk (1) is arranged in a calibration plane (KE) in a measuring volume (11) through which the particles (P) pass during measuring operation of the particle sensor (2). Method according to Claim 1, in which the calibration intensity distribution (10) in the calibration plane (KE) and a measurement intensity distribution (16), in particular a measurement focus, are mapped onto one another in a measurement plane (ME), the measurement plane (ME) being in a measurement volume (11 ) is arranged, which is traversed by the particles (P) in the measuring operation of the particle sensor (2). Method according to Claim 3, in which the calibration disc (1) is arranged in a housing (23) which is separate from the measuring volume (11). Method according to one of the preceding claims, in which the contrast areas (4) on the calibration disc (1) have different surface areas. Method according to one of the preceding claims, in which the contrasting areas are formed by microstructures (4) on the surface of a substrate (3), in particular a wafer. Method according to one of Claims 1 to 6, in which the contrast areas are formed by calibration particles (4), the calibration particles (4) preferably having at least two groups (4a-c) of calibration particles (4) with different optical properties, which in particular consist of different materials are formed. Method according to Claim 7, in which the calibration particles (4) are applied in a statistically distributed manner to the surface of a substrate (3), in particular a wafer, and/or in which the calibration particles (4) are formed by self-structuring on the surface of the substrate (3). will. Method according to one of the preceding claims, in which the calibration disk (1) is automatically rotated and/or automatically displaced when moving in the calibration plane (KE). Particle sensor (2), comprising: a transmitter (5), which has a light source, in particular a laser source (7), for generating a light beam, in particular a laser beam (8), optics (9; 9, 17) for alignment, in particular for focusing the light beam, in particular the laser beam (8), on a calibration plane (KE) to generate a calibration intensity distribution (10), in particular a calibration focus, in the calibration plane (KE), with a calibration disc (1st ) is arranged, are formed on the contrast areas (4) for modulating the intensity (I) of the light beam, in particular the laser beam (8), a movement device (14a; 14b) for moving the calibration disc (1) and/or the calibration intensity distribution (10) in the calibration plane (KE), a receiver (6) which has a detector (12) for detecting at least one intensity signal (l _T , l _R ) of the light beam, in particular of the laser beam (8). passing through the calibration level (KE), and an evaluation device (13) for evaluating the at least one intensity signal (l _T , IR) for calibrating the particle sensor (2) in a calibration mode of the particle sensor (2). Particle sensor according to claim 10, in which the calibration disk (1) is arranged in a calibration plane (KE) which is located in a measuring volume (11) through which the particles (P) pass when the particle sensor (2) is measuring. Particle sensor according to Claim 10, in which the optics (9, 17) have imaging optics (17) for imaging the calibration intensity distribution (10) in the calibration plane (KE) and a measurement intensity distribution (16), in particular a measurement focus, in a measurement plane (ME) one on top of the other, the measurement plane (ME) being formed in a measurement volume (11) through which the particles (P) pass when the particle sensor (2) is in measurement mode. Particle sensor according to one of Claims 10 to 12, in which the transmitter (5) has a housing (23) with an exit window (21a) and in which the receiver (6) has a housing (24) with an entrance window (21b). , between which the measurement volume (11) is formed. Particle sensor according to one of Claims 10 to 13, in which the movement device (14a) is designed to rotate the calibration disc (1) in the calibration plane (KE). Particle sensor according to one of Claims 10 to 14, in which the movement device (14b) is designed to move the calibration disk (1) in the calibration plane (KE). Device, in particular EUV radiation generating device (30), 28 comprising: a measuring chamber (38) to which particles (P) can be supplied, and a particle sensor (1) according to any one of claims 10 to 15 for characterizing the particles (P) in the measuring chamber (38).